Sunan at-Tirmithiy كتاب الصلاة باب ما جاء فى الأذان فى السفر
The Messenger of God, peace and blessings of God be upon him, said, "If anyone feels sleepy while praying[1], he [2] should nap until he feels refreshed, for indeed, if one prays while feeling drowsy, one may [want to] ask for forgiveness from God but curse oneself instead[3]." Footnotes: [1] Some scholars confine this hadīth to the voluntary night prayer and other voluntary prayers, while others generalize it to all obligatory and voluntary prayers. [2] Masculine pronouns are generally used in Arabic as a means for referring to both men and women when the meaning is left general and is not otherwise specified. [3] i.e. he might invoke God against himself instead of invoking Him for himself as a result of being too sleepy to be aware of what he is saying. Click on ḥadīth to read more