Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: I went in to the Prophet (peace and blessings be upon him) and he (peace and blessings be upon him) was praying while sitting with his knees drawn up and wrapped in his garment (ihtiba'), and the buttons of his (peace and blessings be upon him) collar were undone.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11286