It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him): "When the Prophet (peace and blessings be upon him) prayed, he would lay his left foot flat and keep his right foot upright."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from al-A'mash (may Allah have mercy on him) except al-Husayn bin al-Hasan (may Allah have mercy on him), and Umar bin Shabbah (may Allah have mercy on him) is alone in narrating it."
Hadith Referenceالمعجم الأوسط / باب العين / 4450
Hadith Takhrij«أخرجه الطبراني فى «الصغير» برقم: 638»