The Prophet ﷺ was asked whether umrah is obligatory. He replied, "No, but performing it is better."
حدثنا محمد بن عبد الأعلى الصنعاني حدثنا عمر بن علي عن الحجاج عن محمد بن المنكدر
أن النبي ﷺ سئل عن العمرة أواجبة هي قال لا وأن تعتمروا هو أفضل
No references are available.
[Its chain of transmission is daif]. Ahmad narrated it (3/316) from the hadith of Hajjaj bin Artah with this wording, and Ibn Khuzayma criticized it (Hadith 3068). It has a corroborating report (mawquf) with Bayhaqi (4/349), but its chain is daif. This Hajjaj is daif due to the weakness of his memory.