It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) came out of the toilet and food was brought near to him. The Companions offered him water (for ablution), and he (peace and blessings be upon him) said: "I have only been commanded to perform ablution when I stand for prayer." Abu Bakr said: The Prophet (peace and blessings be upon him) informed that Allah Almighty commanded him to perform ablution when he stands for prayer, not upon the entry of a (prayer) time or its ending.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1636