May one wash his hands using the bathroom sink?[1]
Water from the sink:[2] One is to avoid washing his hands for the morning washing, or for bread, using a bathroom sink.[3] However, in a time of need, one may wash his hands inside a bathroom on behalf of the morning washing, or to eat bread.[4] In such a case, one is to only wash his hands in the bathroom, however the saying of the blessing and the drying of his hands is to take place outside the bathroom.[5] In all cases, if one is able to bring the water of the bathroom to outside the bathroom, then one is to do so and wash there.[6] However, initially, one is not to wash with water from the bathroom sink even if he brings it outside.[7]
Toilet water:[8] The above leniency is only with regards to sink water. However, toilet water is invalid for washing hands even if it is clean. [The same applies for any water that has remained in the bathroom for a considerable amount of time and is considered repulsive by people.[9]]
[1] See Shem Mishimon O.C. 9; Kaf Hachaim 4:11; Y.D. 116:73; Os Chaim 43:1; Chazon Ish Netilas Yadayim 24:26; Eretz Tzefvi 1:110-111; Zekan Aaron 1:1; Chelkas Yaakov 1:2-4; 2:162; Minchas Yitzchak 1:60; 4:36; Divreiy Yisrael 1:8; Divrei Yatziv Y.D. 34; Devar Yehoshua 1:76; Yehsuas Moshe 1:31; 2:101; Or Letziyon 1:1; Kinyan torah 1:49; Vayaan Yosef 1:2; Yabia Omer 3:1-2; 9:108; Koveitz Mibeis Levi 7:71; Piskeiy Teshuvos 4:19; 160:19; 171:9
[2] Shem Mishimon O.C. 9; Os Chaim 43:1; Minchas Yitzchak 4:36; Chelkas Yaakov 2:162; Vayaan Yosef 1:2; Yabia Omer 9:108; Piskeiy Teshuvos 4:19; Kaf Hachaim 4:11; Y.D. 116:73 [The Kaf Hachaim ibid differentiates between using the bathroom washing water for washing after use of the bathroom, which is permitted in a time of need, versus for washing in the morning after awakening, or for bread, in which case you say a blessing over it and it is more stringent and the water is invalid. However, it is unclear if this invalidation is even with sink water or only with toilet water. See Devar Yehoshua 1:76; Piskeiy Teshuvos 160:19 footnote 73 who learns that it only applies to water that stayed in the bathroom for a while, and not to sink water]
Background: Although there is no prohibition in doing a Mitzvah within a bathroom [Darkei Teshuvah Yoreh Deah 19:21] nevertheless the question is raised whether the water in a bathroom is able to purify one’s hands as one who enters a bathroom [even without doing his needs] is impurified. Furthermore, perhaps the water in a bathroom is considered repulsive and is invalid for washing just as we rule regarding water that was drunk by a dog.
[3] The reason: 1) As one is not to perform a Mitzvah in an area that is forbidden to study Torah. [Shem Mishimon O.C. 9] As a bathroom is a place of impurity and hence how can washing in it remove the impurity. [Minchas Yitzchak ibid in name of Poskim]
[4] Os Chaim 43:1; Minchas Yitzchak 4:36; Piskeiy Teshuvos 4:19; 160:19 in name of all Poskim ibid
Other opinions: Some Poskim rule that one may not wash his hands for bread using water from a bathroom, being that the water is considered invalid. [Levushei Mordechai Tinyana 182; Possible implication of Ben Ish Chaiy Achreiy-Kedoshim 13 and Kaf Hachaim 4:11]
[5] See Yabia Omer 3:1; Chazon Ish 24:26; Vayaan Dovid 1:1; Az Nidbaru 14:86; Piskeiy Teshuvos 4 footnote 192
[6] Har Tzevi 1:50; Devar Yehoshua 1:76; Vayaan Yosef 1:2; Piskeiy Teshuvos 4:19; 160:19; 171:9
[7] Poskim ibid
[8] Ben Ish Chaiy Achreiy-Kedoshim 13; Kaf Hachaim 4:11 based on that this water is considered repulsive just like water which a pig drank from and which the Poskim rule is invalid [Admur 160:8; M”A 160:7; Bach 160, brought in Beir Heiytiv 160:8], and hence may not be used; Kaf Hachaim Y.D. 116:73; Daas Torah 160:4; Devar Yehoshua 1:76; See Piskeiy Teshuvos 160:19
Other opinions: Some Poskim rule that water from which a dog or pig drank from remains permitted to be used for washing hands for bread [Mamar Mordechai 160:6; 1st and Stam opinion in Adcmur ibid; See Admur ibid who records opinion of Bach as Yeish Poslim] and the same would seemingly apply here regarding bathroom water. [See Kaf Hachaim 4:11]
[9] Devar Yehoshua 1:76; Piskeiy Teshuvos 160:19 footnote 73