Using the bathroom prior to prayer:
After getting dressed [prior to prayer] one should check if he needs to use the bathroom, in order so he does not need to go to the bathroom in the middle of prayer. The Sages said that anyone who wants to accept the yolk of heaven in a complete manner is to use the bathroom [if needed] and then wash and pray.
 Kama 2/8
 Brachos 15a
 See Chida in Kesher Gudal Supplements 1 [in end of Sefer] that this is an addition to the previous reason.