Abdul Rehman b. Abi Yala narrates from his father:
the Holy Prophet (s.a.w.a.) sent Abu Bakr and gave him the flag (on the Day of Khyber), and he returned unsuccessful. Then he (s.a.w.a.) sent Umar and gave him a flag and he returned unsuccessful with the Muslims. Then the Prophet (s.a.w.a.) said I will give the flag to the one who loves Allah and his Prophet (s.a.w.a.) and is beloved of Allah and his Prophet (s.a.w.a.) and he is not the one to flee the battlefield. Then he sent a man to Ali while he was having conjunctivitis, so he put some saliva on his eyes and said: O Allah! Protect him from heat and cold. Then Ali (a.s.) said I never felt heat and cold afterwards.
Sunan Kubra v 7 p 411 Musnad Ahmad v 1 p 99, 133 Fazail al-Sahaba v 2 p 698, 791 Sunan Ibn Majaah trad 117 Al-Mo'jam al-Awsat v 3 p 151, v 6 p 369 Zakhaaer al-Uqba p 90 Riyaaz al-Nazarah v 3 p 133 al-Bedaayah Wa al-Nehaayah v 5 p 452 Majma' al-Zawaaed v 9 p 123 Fath al-Baari Fi Sharh al-Bukhaari v 8 p 255