Narrated Muhammad bin Al-Munkadir: Once Jabir offered namaz with his Izar tied to his back while his clothes were lying besides him on a wooden peg. Somebody asked him, "Do you offer namaz in a single Izar?" He replied, "I did so to show it to a fool like you. Did anyone of us two garments in the life-time of the Prophet?" Saheeh Bukhari Book of namaz: 348.
Narrated Muhammad bin Al Munkadir: I saw Jabir bin 'Abdullah praying in a single garment and he said that he had seen the Prophet praying in a single garment. Saheeh Bukhari, Book of namaz: 349.
Narrated 'Umar bin Abi Salama: I saw the Prophet (s.a.w) offering namaz in a single garment in the house of Um-Salama. He had crossed its ends around his shoulders. Saheeh Bukhari book of namaz: Saheeh Bukhari- Book of namaz: 351.
It can be deduced from the above mentioned ahadith that offering Sunnah in a single garment is Sunnah. However, offering it in two garments such as a shalwar qamiz is afzal too.
NOTE: In the era of the Prophet Mohammad (s.a.w.) people could not afford two garments hence they used to tie a knot around their neck so that the garment would remain in its place (Fathul bari taba salfia-1/467)
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