What is the ruling on the interval between two prayers combined in the time of the first prayer?

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When combining prayers in the time of the first prayer, continuity between them must be observed, meaning the separation between the two prayers must be only a slight separation (tafriq yaseer). If the separation is prolonged, the combination (jam'u) is invalidated. This is because the meaning of combination implies sequence or proximity. If the separation is significant (tafriq katheer), the combination is void, regardless of the reason for the separation, whether it was due to sleep, forgetfulness, occupation, intent, or other reasons. A slight separation does not invalidate the combination because it is unavoidable. The determination of what constitutes a slight versus a significant separation refers back to custom and usage (urf and 'adah); there is no fixed measure for it, though some companions estimated it to be the duration required for ablution (wudu') and prayer. The sound position is that there is no fixed measure, as matters not specified by the Lawgiver cannot be quantified; reference must be made to custom, similar to concepts like 'possession' (ihraz) or 'seizure' (qabd).