Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "When anyone feels drowsy while praying, let him sleep on his bed, for he does not know whether he is praying for himself or against himself."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4729