This issue is to be dealt with under the juristic ruling: ‘What is not regarded as an integral part of something, cannot be offered in making up for missing an integral part of it.’ This is to say, since there is no prostration in the funeral prayer anyway, it is with greater reason that a prostration for forgetfulness is not to be offered to make up for missing an integral element of the prayer.
Az-Zarkashi, an eminent Shafi`i scholar, stated in his book Al-Manthur: “What is not regarded as an integral part of something, cannot be offered in making up for missing an integral part of it.’ Hence, if one fails to perform something that is required in the funeral prayer, one is not to offer a prostration for forgetfulness, because there is no prostration in the funeral prayer.”
However, some scholars have criticized this opinion by saying that sacrificing an animal is not an integral of performing Hajj, yet it is offered as an atonement for violating a restriction of Ihram.
According to Al-Mughni by Ibn Qudamah, an eminent Hanbali scholar, if one has intentionally missed a rak`ah a four-rak`ah prayer, the prayer is invalid. The same applies if a takbirah in the funeral prayer is intentionally missed. However, if missing it was unintentional, one may repeat the prayer as Anas (may Allah be pleased with him) did when he forgot a takbirah in a funeral prayer. And one may also make up for forgetting the performance of the rak`ah or the takbirah by performing them after finishing the prayer, unless a long time has elapsed after the end of the prayer. [If a long time has elapsed, the prayer is to be repeated.] It must also be kept in mind that when making up for the missed rak`ah or takbirah, no prostration of forgetfulness is to be offered.
According to Hashiayat Ad-Dusuqi by Ibn `Arafa Ad-Dusuqi, an eminent Maliki scholar, if the Imam has finished the funeral prayer before performing the fourth takbirah, the followers are not to follow him in doing so. Rather, they are to attract his attention to the mistake he has made by glorifying Allah aloud, so that he may make up for what has been missed. (takbirah) If he realizes what he has done and makes up for the takbirah, they are to follow him until he finishes the prayer with saying as-Salamu alykum. If he does not realize his mistake soon after it was made, the followers should perform the takbirah and finish the prayer on their own. It was also said that if he did not realize the mistake quickly, his prayer would be invalid as well as that of his followers, but the former opinion, i.e., the followers’ performing the missed takbirah on their own, is believed to be the most correct. The followers, in this case, should not discuss the matter with the Imam after finishing the prayer, but Ibn Al-Qasim, an eminent scholar, was of the opinion that they should do so.