Sa'id bin Abi Ayyub (may Allah have mercy on him) narrates: My father (may Allah have mercy on him) narrated to me: I never saw Ubaydullah bin Umar except with the buttons of his shirt undone.
Sa'id (may Allah have mercy on him) narrates: Zuhrah bin Ma'bad al-Qurashi (may Allah have mercy on him) told me: I saw Ibn al-Musayyib (may Allah have mercy on him), Abu Hazim (may Allah have mercy on him) and Muhammad bin al-Munkadir (may Allah have mercy on him); these men would pray with the buttons of their shirts undone.
It has also been reported to us from Ibn Abbas (may Allah be pleased with them both) the like of what is reported from Ibn Umar (may Allah be pleased with them both).
And when that is in prayer, in our view it is to be understood as the case where the neck opening is narrow. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3297