Daraqutni

Asim bin Damrah narrates: We asked Sayyiduna Ali (may Allah be pleased with him) about the prayer of the Noble Prophet (peace and blessings be upon him), so he said: "Who has the strength for it?" We said: We do not have the strength (but please tell us about it), so Sayyiduna Ali (may Allah be pleased with him) informed us: The Noble Prophet (peace and blessings be upon him) would wait until the sun had moved away from its rising place as far as it moves away from its setting place at the time of Asr, at that time he would perform two rak'ahs of prayer. Then he would wait until the sun had moved away from its rising place as far as it moves away from its setting place (i.e., when the time of decline had come), at that time the Noble Prophet (peace and blessings be upon him) would perform four rak'ahs of Sunnah before the Zuhr prayer after the decline, after Zuhr he would perform two rak'ahs of Sunnah, and then he would again perform four rak'ahs before (the prayer).