Abu Dawud Book 2, Hadith Number 1363.
Chapter : The commandment of observing moderation in praying.
Narated By ‘Aisha : The Apostle of Allah (PBUH) said: Choose such actions as you are capable of performing, for Allah does not grow weary till you do. The acts most pleasing to Allah are those which are done most continuously, even if they mount to little. Whenever he began an action, he would do it continuously.