“Fasting in winter is the cold booty.”

Narrated by Aḥmad (no. 18959), al-Bayhaqī in al-Sunan al-Kubrā (no. 8454) and the wording is theirs, al-Tirmidhī (no. 797), Ibn Khuzaymah (no. 2145), and Ibn Abī Shaybah (no. 9741), from the narration of ʿĀmir ibn Masʿūd — may Allah have mercy on him.
Authenticated in Ṣaḥīḥ al-Jāmiʿ (no. 3868) and Silsilat al-Aḥādīth al-Ṣaḥīḥah (no. 1922).

Brief Explanation of the Hadith

The Prophet — may Allah’s peace and blessings be upon him — clarified that among the manifestations of Allah’s mercy to His servants is that He has appointed times in which acts of worship become easier, while the reward and recompense therein are multiplied — as an encouragement for them to strive and as an exhortation to seize such times.
Among such times is what the Prophet — may Allah’s peace and blessings be upon him — informed of: that fasting in the winter is from the “cold spoils.” The meaning is that fasting in the season of winter resembles a war booty attained without combat or hardship; for there is no hunger due to the length of the day, nor thirst from its heat. Thus, the servant performs the act of worship with ease and facility, yet is granted immense reward and abundant recompense without any diminishment — and Allah’s bounty is vast.
In this ḥadīth: an encouragement to increase fasting during the winter season, and to seize the times of virtue. It also highlights the vastness of Allah’s mercy, and the greatness of His generosity and bounty.