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