Hadith: I said: O Messenger of Allah, to whom should I be dutiful? The Prophet (may Allah's peace and blessings be upon him) said: Your mother, then your mother, then your mother, then your father, then the closest next of kin, and so on

عن بهز بن حكيم، عن أبيه، عن جده، قال: قلتُ يا رسول الله: مَن أَبَرّ؟ قال: "أُمَّك، ثم أُمَّك، ثم أُمَّك، ثم أَباك، ثم الأقربَ، فالأقربَ".
[حسن] - [رواه أبو داود والترمذي وأحمد]

Bahz ibn Hakīm reported from his father, who reported from his grandfather that he said: "I said: 'O Messenger of Allah, to whom should I be dutiful?' The Prophet (may Allah's peace and blessings be upon him) said: 'Your mother, then your mother, then your mother, then your father, then the closest next of kin, and so on.'"
Hasan/Sound. - [At-Tirmidhi]

Explanation

This Hadīth encourages maintaining good relationship with one's relatives. The worthiest of this good relationship is one's mother, then one's father, then the closest next of kin, etc. Mothers are the worthiest of one's kindness because of the troubles they went through, and because of their compassion and service. They have the merit of conception, nursing, and raising their children. Pregnancy is a hard experience, and so is giving birth. In this regard, Allah, the Almighty, says: {His mother carried him with hardship and gave birth to him with hardship} [Sūrat al-Ahqāf: 15]. As the mother is given precedence over the father, it follows that she is also given precedence over everyone else other than the father. Part of being dutiful to one's parents is to spend on them.