What is the evidence for the rule: “What is established in the voluntary prayer is established in the obligatory prayer”?

The questioner : Is there a rule, O Ghanim, in this chapter? What is it?

The Shaykh : What is established in the voluntary prayer, and what is established in the voluntary prayer is established in the

The questioner : except with evidence. Now, is there evidence from the words of the Companions for this rule?

The Shaykh : Abd al-Rahman.

The questioner : The action and statement of the Companions that the Prophet (peace and blessings be upon him) used to pray.

The Shaykh : He would perform the witr prayer on his mount.

The questioner : Toward his mount, wherever it faced, or toward a direction other than the qiblah, for the obligatory prayer.

The Shaykh : Wherever his face was directed, except that he did not pray the obligatory prayer on it.

The questioner : Welcome.

The Shaykh : Now, this indicates that what is established in the voluntary prayer is established in the obligatory prayer; otherwise, the exception would have no benefit. Understood? Good.