Musannaf Ibn Abi Shayba 3:1724

Abu Bakr narrated to us, Hammad bin Abd al-Rahman narrated to us, from Zuhayr, from Jabir bin Yazid bin Murrah, from Suwayd bin Ghafalah, who said: "If a man enters while the mu'adhdhin is performing the iqamah for the prayer, he should stand as he is if he wishes, for that is easier for the elderly man." And 'Amir said: "There is no harm in it."