According to the Noble Hadith, the Prophet (peace be upon him) would recite the Shahadah while looking towards the sky after performing wudu, but there is no evidence of him pointing towards the sky with a finger while doing so. However, the Prophet (peace be upon him) would sometimes lift his finger and point towards the sky when reciting the Shahada while delivering a Khutbah. Therefore, some Fuqaha (scholars) have deemed it mustahabb to point to the sky while looking towards the sky.
Based on this, if a person points towards the sky while reciting the Shahadah after performing wudu, they should not be told to refrain from it. However, it should also not be considered obligatory or necessary to point to the sky while reciting the Shahadah after performing wudu.
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دلائل:
الھندیة: (8/1، ط: دار الفکر)
وأن تقول: بعد الفراغ من الوضوء سبحانك اللهم وبحمدك أشهد أن لا إله إلا أنت أستغفرك وأتوب إليك وأشهد أن لا إله إلا الله وأشهد أن محمدا عبده ورسوله
حاشیة الطحطاوی علی مراقی الفلاح: (ص: 77، ط: دار الکتب العلمیة)
"والإتيان بالشهادتين بعده" ذكر الغزنوي أنه يشير بسبابته حين النظر إلى السماء۔۔۔الخ
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.