(a) It is transmitted from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he saw no harm in a man offering voluntary prayer in the same place, or (the narrator said:) he saw him doing so. This doubt is from the narrator Ali (bin Thabit).
(b) It has been narrated to us from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he distinguished in this matter between the imam and the follower, considering it disliked (makruh) for the imam to do so while holding it permissible for the follower. Its chain is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3054