Hadith #1635 (Jami' al-Tirmidhi)
Book: The Book on Virtues of Jihad
Narrator(s): Abdul Wahid bin Wasil عبد الواحد بن واصل أبو عبيدة الحداد, Ibn Sirin محمد بن سيرين الأنصاري

That the Messenger of Allah (ﷺ) said: “Whoever develops some gray hair in the cause of Allah, it shall be a light for him in the Day of Judgement.
 This Hadith is Hasan Sahih Gharib. As for Haiwah bin Shuraih, (the remainder of his name is) Ibn Yazid Al-Himsi.

Source: https://parallelquran.com/hadith/jami-al-tirmidhi-1635/