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
(a) Nu'man bin Abi Ayyash (may Allah have mercy on him) says that we sought permission from the Messenger of Allah (peace and blessings be upon him) to lean and take support in prayer, and he (peace and blessings be upon him) granted us the concession that "a person may take support with his elbows on his knees and thighs."
(b) Likewise, Sufyan al-Thawri (may Allah have mercy on him) narrates from Sumayy (may Allah have mercy on him), and he from Nu'man (may Allah have mercy on him), that the Companions of the Prophet (peace and blessings be upon him) (may Allah be pleased with them) complained; he mentioned this hadith as mursal.
Imam al-Bukhari (may Allah have mercy on him) says that it is more correct that it is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2721