"Truncated (Incomplete) Durood is Not Acceptable to Rasulullah (peace and blessings be upon him):

Reciting Durood Sharif is an important act of worship in Islam. The Qur'an and Hadith strongly encourage believers to send salutations (Durood) upon the Messenger of Allah, Muhammad (peace be upon him). However, it is essential to maintain completeness when reciting Durood. According to Hadith, a truncated or incomplete Durood is not acceptable to the Prophet (PBUH) and does not carry the full blessings.

Evidences from Hadith:

In the Holy Qur’an, Allah says, “Indeed, Allah and His angels send blessings upon the Prophet. O you who believe! Send blessings upon him and send salutations of peace.” (Surah Al-Ahzab: 56, Al-Qur’an)

After this verse was revealed, the companions (Sahabah) asked, "O Messenger of Allah (PBUH), how should we send Durood upon you?" In response, the Prophet (PBUH) said, "Say: Allahumma salli ‘ala Muhammad wa ‘ala Aali Muhammad (O Allah, send blessings upon Muhammad and upon the family of Muhammad)."

Then he added, “Beware! Do not send a ‘truncated Durood’ upon me.”

The companions asked: “What is that (a truncated Durood)?”

The Prophet (PBUH) replied: “To send blessings upon me while excluding my Ahlul Bayt (Ali, Fatimah, Hasan, and Husayn) – such as saying only: ‘Allahumma salli ‘ala Muhammad’ and then stopping there.”

Sources:

Jarubay al-Wilayah (p. 154), Majma'ul Bayan, Vol. 8 (p. 369), Muraja’at (p. 66), Musnad Ahmad, Vol. 5 (p. 353), Zakhair al-‘Uqba (p. 19), Yanabi‘ al-Mawaddah (p. 7), Kanz al-‘Ummal, Vol. 1 (p. 124), Sawaiq al-Muhriqah (p. 87), Fadhail al-Khamsa, Vol. 1 (p. 209)

Hazrat Abu Mas‘ud (RA) narrated: "The Messenger of Allah (PBUH) used to teach us how to send blessings upon him. We asked: 'O Messenger of Allah, how should we do that?'" He replied: "Say: Allahumma salli ‘ala Muhammad wa ‘ala Aali Muhammad." (Sahih al-Bukhari and Sahih Muslim)

This Hadith clearly shows that stopping at “Allahumma salli ‘ala Muhammad” is an incomplete Durood, because it does not include blessings upon Aal-e-Muhammad (i.e., the family and followers of the Prophet).

Consequences of an Incomplete Durood:

The Prophet (PBUH) does not accept Durood unless it is complete. A Hadith states, "Whoever sends blessings upon me but excludes my Ahlul Bayt, that Durood does not reach me." (Mishkat, Al-Kawthar)

Incomplete Durood reduces the spiritual merit of the worship and is considered against proper etiquette (adab).

Conclusion:

When sending Durood upon the Prophet (PBUH), it is best and proper to follow the complete form as taught in the Hadith. Incomplete Durood is disrespectful and not acceptable to the Messenger of Allah (PBUH).