It is narrated from Umm al-Mu'mineen Sayyida Aisha (may Allah be pleased with her) that the Prophet Muhammad (peace and blessings be upon him) said: "When any of you becomes drowsy while in prayer, let him turn away (from prayer), for he does not know, perhaps he may begin to supplicate for something bad instead of seeking forgiveness."
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 31
Hadith Takhrijمسند احمد (42/462، رقم: 25699) وقال الارنوؤط: اسناده على شرط الشيخين وصححه ابن خزيمة، رقم: 907