Performing One Rakaat Sitting And Another Standing

 In Fardh Salah to perform Salah standing is also Fardh, therefore, one will only be absolved from this when he is genuinely unable to stand. Thus, in a case when one is unable to stand and he performed the first Rak’ah sitting and then became able, he should perform the second Rak’ah standing and if he offered the first Rak’ah standing and then became unable, he may perform the second Rak’ah sitting.

In Nafl Salah it is permissible to perform the Salah sitting without a reason. If a person performed the first Rak’ah of Nafl Salah standing and then sat down in the second Rak’ah it is permissible, however it is inappropriate to perform one Rak’ah standing and one rak’ah sitting without any reason.