Is one who does not fully adhere to the Sunnah of the Messenger, may Allah bless him and grant him peace, such as the growing of a beard, sinful?

Obeying the Messenger, may Allah bless him and grant him peace, is obeying Allah the Almighty, and his Sunnah is a revelation from Allah. Whoever follows his Sunnah will attain Allah's love and forgiveness. His Sunnah is divided into the obligatory, which must not be contravened, such as growing a beard, and the recommended, such as performing the night vigil (Qiyam al-Layl) and Duha prayer. It is important to persist in acting upon it to attain reward and Allah's love.