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.
Jami` at-Tirmidhi Book 22, Hadith 17
← Previous Hadith "O Ka'b bin Murrah! Relate (something) to is from the Messenger of Allah (ﷺ), and be cautious. He said: 'I heard the Prophet (ﷺ) say: "Whoever develops some gray hair in Islam, it shall be a light from him on the Day of Judgement."

There is something on this topic from Fadalah bin 'Ubaid and 'Abdullah bin 'Amr. The narration of Ka'b bin Murrah was reported like this from Al-A'mash, from 'Amr bin Murrah.

This Hadith is been reported from Mansur, from Salim bin Abu Al-Ja'd, and he included a man between him and between Ka'b bin Murrah in the chain. He is called: "Ka'b bin Murrah," and he is called: "Murrah bin Ka'b Al-Bahzi," and the one known among the Companions of the Prophet (ﷺ) is Ka'b bin Murrah Al-Bahzi, he reported some Ahadith from the Prophet (ﷺ).

Jami` at-Tirmidhi Book 22, Hadith 16 Next Hadith → That the Messenger of Allah (ﷺ) said: "The forelocks of horses contain good until the Day of Judgement. The horse is for three (purposes): It is for one man a reward, and it is for one man a shelter (from poverty), and it is from one man a burden. As for the one who acquires it for the cause of Allah and then prepares it for that; it is for him a reward, nothing disappears into its stomach except that Allah writes it for him as a reward."

This Hadith is Hasan Sahih. Malik bin Anas narrated similar to this Hadith from Zaid bin Aslam from Abu Salih from Abu Hurairah, from the Prophet (ﷺ).

Jami` at-Tirmidhi Book 22, Hadith 18