Abbas bin Sahl al-Sa'idi, reporting from Abu Humayd (may Allah be pleased with him) the description of the prayer of the Messenger of Allah (peace and blessings be upon him), says: When he (peace and blessings be upon him) prostrated, he kept his legs apart and kept his belly away from his thighs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2712
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When one of you prostrates, let him not spread his arms like a dog, and let him keep his thighs together."
(b) Keeping the legs apart is more fitting for prostration. And Allah Almighty knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2713
It is narrated from Sayyiduna al-Bara' (may Allah be pleased with him) that, while describing the manner of prostration, he spread his hands, kept his back raised, kept his belly away from the ground and his arms away from his sides, and said: "This is how the Messenger of Allah (peace and blessings be upon him) used to prostrate."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2714
The complete hadith about sitting to relieve oneself is reported from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both). In it he said: Perhaps you are one of those who pray upon their haunches. The narrator says: I said: By Allah, I do not know. He said: He meant the one who, in prostration, does not raise himself off the ground, that is, he prostrates while clinging to the ground.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2715
Shaqiq (may Allah have mercy on him) says that Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: None of you should ever prostrate leaning on his buttocks or hip, nor prostrate lying down, for when a person prostrates well, all his bones prostrate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2716