It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Companions complained to the Prophet (peace and blessings be upon him) that when they prostrated with their arms spread apart, it was a strain for them. He (peace and blessings be upon him) said: "Take support from your knees." Shu'ayb added in his narration that Ibn Ajlan (may Allah have mercy on him) said: 'Ista'inu bir-rukab' (take support from the knees) means that a person rests his elbows on his knees when he prolongs the prostration and becomes tired.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2720