(a) Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) permitted Abd al-Rahman bin Awf and al-Zubayr bin al-Awwam (may Allah be pleased with them both) to wear silk shirts while travelling because of itching, or because of some other ailment.
(b) Qatadah (may Allah have mercy on him) said that the Prophet (peace and blessings be upon him) gave permission to wear silk shirts during a campaign.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6078