Hadith: I saw ‘Ammār ibn Yāsir perform ablution, and he ran his fingers through his beard. It was said to him (or he said: I said to him): Do you run your fingers through your beard? He replied: What is there to prevent me if I have seen the Messenger of Allah (may Allah's peace and blessings be upon him) run his fingers through his beard?

عن حسان بن بلال قال: رأيت عمار بن ياسر رضي الله عنه توضأ فخَلَّلَ لِحْيَتَهُ، فقيل له: -أو قال: فقلت له:- أَتُخَلِّلُ لِحْيَتَك؟ قال: «وما يمنعُني؟ ولقد رأيت رسول الله صلى الله عليه وسلم يُخَلِّلُ لِحْيَتَه».
[صحيح] - [رواه الترمذي وابن ماجه]

Hassān ibn Bilāl reported: I saw ‘Ammār ibn Yāsir perform ablution, and he ran his fingers through his beard. It was said to him (or he said: "I said to him"): "Do you run your fingers through your beard?" He replied: "What is there to prevent me if I have seen the Messenger of Allah (may Allah's peace and blessings be upon him) run his fingers through his beard?"
[Authentic hadith] - [Narrated by Ibn Majah - Narrated by At-Termedhy]

Explanation

Hassān ibn Bilāl informs us that he saw ‘Ammār ibn Yāsir run his fingers through his beard while performing ablution. He asked him about it because it surprised him as he had not known about it before seeing ‘Ammār ibn Yāsir do this. "What is there to prevent me if I have seen the Messenger of Allah (may Allah's peace and blessings be upon him) run his fingers through his beard?" ‘Ammār (may Allah be pleased with him) replied by saying that nothing prevented him from doing so as he had seen the Prophet (may Allah's peace and blessings be upon him) himself do it. Takhlīl al-Lilyah (running your fingers through your beard) can be performed in two ways: 1. Taking a handful of water, placing it under your beard, and then rubbing it so that the water goes through it. 2. Taking a handful of water and running your fingers through your beard like a comb.